The field of the disclosure relates generally to the transmission of electromagnetic radiation and more particularly to methods and apparatus for reducing loss of such radiation incident on dielectric materials at grazing angles.
When electromagnetic energy is incident upon a dielectric interface at a shallow angle, very little power actually enters the dielectric, particularly for polarizations perpendicular to the plane of incidence (TE). For example, when a dielectric having a dielectric constant of 4.86 (ZnS) is used with incident electromagnetic energy striking the dielectric at 80 degrees off normal, only 30% of the electromagnetic energy actually enters the dielectric. The remaining 70% of the energy is reflected away.
Several conventional anti-reflection techniques are known. These known techniques include dielectric stacks, matching layers and gradients that reduce overall polarizations. Low dielectric constant materials are needed for all of these techniques, although the techniques are equally applicable for various materials and frequencies.
In one known technique, the dielectric constant of a dielectric interface (such as, but not limited to, an RF radome, an IR window, or optical window) is graduated from that of free space to that of a dielectric material being matched. This type of graduation provides transmission over a relatively broad band of wavelengths. In a second known technique, the thickness and dielectric constant of a matching layer is selected so that a reflection from the incident surface of the matching layer exactly cancels that from the interface between the matching layer and the dielectric material being matched, thus producing maximum transmission of electromagnetic radiation into the dielectric material being matched. This matching by cancellation is performed using an off angle equivalent of a quarter wave transformer. Since it is a cancellation technique, the thickness of the dielectric layer depends upon the wavelength (frequency) of operation.
Other finite dielectric thickness techniques are known that cancel reflections from both interface sides of a dielectric material being matched. These techniques, such as those commonly used in radomes, set strict restrictions on the dielectric thickness being matched and do not necessarily maximize power transfer into a dielectric material.
The first two known solutions presented above depend upon materials having dielectric constants that are lower than those of the dielectric material being matched. However, low dielectric constant materials are inherently fragile and cannot be easily engineered. At extreme grazing angles, all dielectric coatings fail and reflect strongly as there are relatively few materials having extremely low dielectric constants and those that do have very poor physical characteristics. The quarter wave technique is very narrow banded and depends upon the availability of a material that must have a low dielectric constant that can be engineered to a particular value. In addition, to have good transmission characteristics, the materials must also have very low loss. As such, there is still an unmet need for coatings and materials that overcome the above described shortcomings.